The present invention relates generally to a method for synthesizing siloxane polymers, and more particularly, to a process for preparing block diorganosiloxane copolymers.
The synthesis of multiple sequence organopolysiloxane block copolymers is known. As described by U.S. Pat. No. 3,578,726, issued to Bostick, such polymers are prepared by initially interacting a silane, having at least two hydrolyzable radicals attached to silicon, with a polydiorganosiloxane having at least one terminal alkali metal silanolate radical. The polydiorganosiloxane is derived from a diorganocyclotrisiloxane and a pre-formed alkali metal silanolate-hexamethylphosphoramide complex. This synthesis is dependent on the formation of the alkali metal silanolate complex, and requires the polydiorganosiloxane to have a terminal alkali metal silanolate, both of which provide inherent limitations.
In one known synthesis of diorganosiloxanes, a phenyl prepolymer is first prepared by mixing octamethylcyclotetrasiloxane (SiMe.sub.2 O).sub.4 and octaphenylcyclotetrasiloxane (Si.phi..sub.2 O).sub.4 under argon (Ar). In the above formulas and throughout the rest of this disclosure, Me represents a methyl (--CH.sub.3) radical, Et represents an ethyl (--C.sub.2 H.sub.5) radical, Vi represents a vinyl (--CH.dbd.CH.sub.2) radical, and .phi. represents a phenyl (--C.sub.6 H.sub.5) radical. A catalyst, Me.sub.4 N.sup.+- (O SiMe.sub.2).sub.n OH (N-catalyst), where n is approximately 4, is added in increments, resulting in the formation of a prepolymer. The prepolymer is then re-equilibrated with (SiMeViO).sub.4, a dimethylaminosilyl-terminated oligomer Me.sub.2 N(SiMe.sub.2 O).sub.3 SiMe.sub.2 NMe.sub.2, and N-catalyst. The reaction product is finally condensed with HO(SiMe.sub.2 O).sub.n H, where n is from 15-21 inclusive. The catalyst, however, is used in the production of the prepolymer, and additionally in the reaction of the prepolymers with the dimethylaminosilyl-terminated oligomer. A significant limitation of this synthesis is that a very large percentage of the initial diorganocyclosiloxane materials is not converted to polymers. The prepolymer that is formed has an exceptionally large molecular weight because the siloxane starting materials combine in an unchecked fashion. The result is creation of a very high molecular weight polymer, with a large percentage of initial reactants wasted. A second limitation of this synthetic sequence is the time for the process to proceed to completion. Reportedly, this sequence can take upwards of two weeks.
It has now been discovered that polydiorganosiloxane block copolymers can be prepared in a much shorter time than previously known, and with a much higher synthetic yield, while minimizing unreacted starting materials waste. The starting materials are essentially all synthesized to the final product, in comparison to existing known synthesis methods wherein up to seventy percent of the starting materials are wasted.